Building TRUST builds on insights gained from TRUST (1R01MH099001), an implementation science research grant that uncovered a high risk men who have sex with men (MSM) hotspot subcohort of highly marginalized men disproportionately represented in later waves of respondent-driven sampling (RDS)(applied as a public health implementation tool) with a high HIV prevalence (44%), low uptake of clinical engagement and treatment as prevention (TasP), low levels of disclosure of HIV and/or sexual orientation, high incidence rates of both HIV infection (13.9 per 100 person- years) and sexually transmitted infections (STI) (gonorrhea and chlamydia) indicative of unprotected anal intercourse, and ego-network data documenting high rates of risky sexual positioning and low rates of serosorting despite knowledge of HIV status. Building TRUST posits that within the treatment and prevention cascade, there are cascade-specific interpersonal, network and structural barriers amendable to targeted intervention(s) that can favorably impact measurable outcomes of efficiency. Building TRUST, framed in the 2014 WHO Consolidated Guidelines on HIV Prevention, Diagnosis, Treatment and Care for Key Populations especially biomedical prevention approaches, employs the Social Ecological Model (SEM) to longitudinally collect individual, interpersonal and environmental data targeting this hotspot cohort and integrates innovative phylodynamic modeling to gain fundamental insights about the drivers of ineffective prevention and treatment outcomes. The 3 aims of Building TRUST are: Specific Aim 1: Framed in the Social Ecologic Model, explore individual (e.g. disclosure), interpersonal (e.g. social/sexual network) and environmental (e.g. legalized stigma- antigay laws) to identify barriers and facilitators of effective engagement in the TasP cascade among HIV- infected MSM. Outcomes: A multidimensional understanding of barriers to effective engagement in TasP indexed by objective biomarkers (e.g. HIV viral load). Specific Aim 2: In the face of suboptimal uptake of behavior change among HIV-uninfected MSM, investigate the impact of introducing pre-exposure prophylaxis (PrEP) (newly adopted by the Government of Nigeria) at the individual (e.g. uptake), interpersonal (e.g. social support, sexual partner), and environmental (e.g. social capital and stigma) levels. Outcomes: A multidimensional understanding of barriers to effective engagement in PrEP indexed by objective biomarkers cross compared to data from Aim 1 to guide intervention strategies. Specific Aim 3: Characterize HIV transmission networks within the MSM community by applying phylodynamic epidemiological analyses linked to bio-behavioral and RDS and ego-network data from Aim 1 and among high-risk HIV-uninfected MSM, incident HIV infections from Aim 2 follow-up. Outcomes: Enhanced understanding of how HIV transmission occurs within and across MSM sexual networks leading to actionable interventions tailored to interdict forward transmission.